The present invention is related to a bushing screen for thermally conditioning the molten glass flowing down to an orifice plate in a glass fiber reinforcement manufacturing operation.
In a glass fiber reinforcement manufacturing operation, a bushing is used to supply a plurality of molten glass streams, which are attenuated and drawn out to form fibers. Usually the bushing is in the form of a metal box having an elongate, generally rectangular shape. This box is commonly referred to as a bushing body. The bushing body is defined in part by opposing end plates and side plates. The upper bushing body has an opening through which a source of glass is supplied. The opening is commonly referred to as the throat portion of the upper bushing body. The bottom of the bushing body is defined by a bottom wall that is typically referred to as a tip plate. The bushing body is heated to a desired temperature by its own electrical resistance by passing electric current through the bushing body. The current is supplied to the end plate through an electrical terminal or ear that is welded to the end plate. To help retain the heat and give support to the bushing body, refractory insulation is provided about the end plates and side plates. The glass is conditioned to a desired temperature through thermal interaction with the resistance heated components of the bushing body. The bottom wall has a plurality of orifices through which the molten glass flows, forming heat-softened glass streams.
A screen is located within the upper portion of the upper bushing body. The screen extends in a longitudinal or lengthwise direction and is attached to the end plates. Prior to the 1980s, the primary role of the screen was to serve as a xe2x80x9cstone catcherxe2x80x9d and prevent glass contact refractory and glass inclusions from reaching the tip plate and resulting in problems with heat patterns and process interruptions. At that time, screens were flat. The end plates, side plates, and tip plate were the principle heat sources and contact surfaces to achieve thermal interaction between the bushing body and the glass. However, as tip counts (the number of orifices in the tip plate) and bushing bodies became larger and nominal throughputs significantly increased, the glass residency time and the intimacy of interaction between the incoming glass and the bushing body structure diminished substantially. It was necessary to find ways to supplement the thermal capacity of the bushing and improve its capability to influence the thermal profile of the glass within the bushing body.
It had become obvious that the screen could serve as a heating element to help refine internal glass temperatures. In the 1980s, more innovative V-shaped and W-shaped screen configurations began to replace the previously flat screens. The increased surface area of the newer shapes increased the amount of thermal interaction with the glass. In some cases, screen hole densities were reduced to increase current flow and screen operation temperatures. It was also observed that the V-shaped and W-shaped screens provided additional benefits with regard to internal glass flow and mixing. The folds and slanted surfaces of the new screens diverted glass flows toward the front and rear portions of the bushing tip plates and improved thermal homogeneity and tip section heat patterns in these directions.
Within the same time period, the multiple density (MD) screen concept was introduced to improve flow patterns and thermal mixing in the lengthwise dimension of the bushing. To implement the MD concept, a perforated area within the center portion of the bushing screen is reduced through changes in hole density and/or hole diameter. The resistance to flow is increased and a portion of the warmer glass that enters the center of the bushing is diverted toward the ends of the bushing. This hotter glass mixes with the cooler glass entering near the ends of the bushing and flows more readily through less flow resistant portions of the screen. This mixing action delivers a glass that is warmer and more thermally homogeneous to the end areas of the tip plate section that had previously demonstrated cooler forming cones and cooler fiber forming conditions.
Both the multiple density and folded screen concepts have been successfully implemented on a wide scale within direct melt bushing systems and have been very beneficial to bushing heat pattern uniformity and have assisted in achieving unprecedented levels of operational efficiency. However, there is still room for further tip section heat pattern improvement in bushings with folded screens. A significant amount of the current to heat internal screens is delivered through the bushing end plates. The portions of the end plates that are located above the location of the screen attachment have a much lower current density than the portion of the end plates that are located below the location of the screen attachment. This creates a current blind spot or an electrically deficient region. The average operating temperature of the end plates in these areas is substantially diminished. This reduces the overall effectiveness of the end plate as a bushing body heating element.
What is needed is an internal screen configuration that maintains the surface area heating and flow diverting benefits of the folded configurations and maximizes the heating capabilities of the bushing end plates. Increasing the heating capabilities of the bushing end plates will help insure delivery of a more thermally homogeneous glass to the bushing tip section.
The present invention is related to an internal screen configuration that improves the tip section heat pattern of a glass fiber bushing system by improving the internal glass flow patterns within a bushing body and maximizing the effectiveness of the end plate as a glass-heating source. One embodiment of the invention is directed toward a screen for use in a glass fiber bushing system. The screen comprises an electrically conductive elongate inner screen plate. The inner screen plate has at least one elongate fold therein and at least two elongate divergent surfaces on opposing sides of the fold. Each divergent surface has an upper end and opposing edges tapering downward from the upper end. The upper ends of the divergent surfaces diverge from one another. Each of the divergent surfaces has a plurality of holes therein. The screen further comprises at least two electrically conductive outer screen plates. The outer screen plates are attached to the edges of the divergent surfaces and extend between the divergent surfaces. Each of the outer screen plates has a plurality of holes therein.
Another embodiment of the invention is directed toward a glass fiber bushing system comprising a bushing body and a screen within the bushing body. The bushing body has opposing end plates. The screen comprises opposing ends. Each of the ends has an upper portion and a lower portion. The upper portion of each of the ends is attached to one of the end plates. The lower portion of each of the ends is spaced apart from the end plates.
Another embodiment of the invention is directed toward a glass fiber bushing system comprising an electrically conductive bushing body and a screen. The bushing body comprises opposing end plates, elongate side plates extending between the end plates, and a tip plate extending between the end plates and the side plates. An opening atop the bushing body defines a throat portion of the bushing body. Opposing electrically conductive terminals are connected to the end plates. An electrically conductive screen is located within the throat portion of the bushing body. The screen comprises an elongate inner screen plate. The inner screen plate has at least one elongate fold therein and at least two elongate divergent surfaces on opposing sides of the fold. Each divergent surface has an upper end and opposing edges. The upper ends of the divergent surfaces diverge from one another. Each of the divergent surfaces has a plurality of holes therein. A pair of opposing outer screen plates are attached to the edges of the divergent surfaces and extend between the divergent surfaces. Each of the outer screen plates has a plurality of holes therein. Each of the outer screen plates has an upper end attached to one of the end plates of the bushing body.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.